1. Field of the Invention
The present invention relates to a method and an apparatus for encoding a video signal comprising a sequence of pictures and, in particular, to an improvement in hybrid codecs such as H.264/AVC including motion compensated prediction and coding of the prediction residual.
2. Description of Prior Art
The most successful class of video compression designs are called hybrid codecs such as H.264/AVC, described in ITU-T Recommendation H.264 & ISO/IEC 14496-10 AVC. Advanced Video Coding for Generic Audiovisual Services. 2003. Motion-compensated prediction and coding of the prediction residual are the fundamental building blocks of these codecs. The operation of a hybrid video encoder involves the optimisation of many decisions to achieve the best possible trade-off between rate and distortion given constraints on delay and complexity. Because of the use of motion-compensated prediction, all these decisions typically depend on each other for many pictures of a coded sequence.
There has been a large amount of work on optimisation problems in hybrid video coding in the past. One particular focus has been on Lagrangian optimisation methods. Such methods are described in: H. Everett III. Generalized Lagrange Multiplier Method for Solving Problems of Optimum Allocation of Resources. Operations Research, 11:399-417, 1963; Y. Shoham and A. Gersho. Efficient Bit Allocation for an Arbitrary Set of Quantizers. IEEE Transactions on Acoustics, Speech and Signal Processing, 36:1445-1453, September 1988”; P. A. Chou, T. Lookabaugh, and R. M. Gray. Entropy-Constrained Vector Quantization. IEEE Transactions on Acoustics, Speech and Signal Processing, 37(1):31-42, January 1989”; and G. J. Sullivan and R. L. Baker. Rate-Distortion Optimized Motion Compensation for Video Compression Using Fixed or Variable Size Blocks. In Proc. GLOBECOM' 91, pages 85-90, Phoenix, Ariz., USA, December 1991”.
Work on considering the dependencies between the various encoding decisions has focused on modelling these dependencies by trellises which allows the use of dynamic programming methods. Bit-allocation to DCT coefficients was proposed by Ortega and Ramchandran for MPEG-2 Video in: K. Ramchandran, A. Ortega, and M. Vetterli. Bit Allocation for Dependent Quantization with Applications to Multiresolution and MPEG Video Coders. IEEE Transactions on Image Processing, 3(5):533-545, September 1994, and a version that handles the more complex structure of the entropy coding of H.263 has been developed in: J. Wren, M. Luttrell, and J. Villasensor. Trellis-Based R-D Optimal Quantization in H.263+. IEEE Transactions on Circuits and System for Video Technology, 1998. The selection of other coding parameters such as motion vectors and macroblock modes has been optimised in: A. Ortega, K. Ramchandran, and M. Vetterli. Optimal Trellis-Based Buffered Compression and Fast Approximations. IEEE Transactions on Image Processing, 3(1):26-40, January 1994”; T. Wiegand, M. Lightstone, D. Mukherjee, T. G. Campbell, and S. K. Mitra. Rate-Distortion Optimized Mode Selection for Very Low Bit Rate Video Coding and the Emerging H.263 Standard. IEEE Transactions on Circuits and Systems for Video Technology, 6(2):182-190, April 1996; J. Lee and B. W. Dickinson. Joint Optimization of Frame Type Selection and Bit Allocation for MPEG Video Coders. In Proceedings of the IEEE International Conference on Image Processing, volume 2, pages 962-966, Austin, Tex., USA, November 1994; M. C. Chen and A. N. Willson. Rate-Distortion Optimal Motion Estimation Algorithm for Video Coding. In Proceedings of the IEEE International Conference on Acoustics, Speech and Signal Processing, volume 4, pages 2096-2099, Atlanta, Ga., USA, May 1996; and G. M. Schuster and A. K. Katsaggelos. A Video Compression Scheme with Optimal Bit Allocation Among Segmentation, Motion, and Residual Error. IEEE Transactions on Image Processing, 6(11):1487-1502, November 1997.